Within the field of memories, there is continuing interest in finding ways to increase the storage density and speed. As the personal use of small devices gain popularity, the memory of these equipments has to be modified to match the function and design of these small devices. Particularly, as more and more data needs to be stored in the memory, the memory needs to have the capacity and speed to handle such demand.
The discovery of new phenomena of magnetoresistive (MR) and giantmagnetoresistive (GMR) effect provided a significant advancement in the field of memory technology. This phenomena demonstrated that resistance of multilayer thin film comprised of ferromagnetic layers sandwiching a conducting layer can change significantly depending on the direction of an external magnetic field.
GMR is observed in magnetic metallic layered structures in which it is possible to orient the magnetic moments of the ferromagnetic layers relative to one another. One such type magnetic metallic layered structure consists of a stack of four magnetic thin films: a free magnetic layer, a nonmagnetic conducting layer, a magnetic pinned layer and an exchange layer. Magnetic orientation of the pinned layer is fixed and held in place by the exchange layer. By applying an external magnetic field, the magnetic orientation of the free layer may be changed with respect to the magnetic orientation of the pinned layer. The change in the magnetic orientation generates a significant change in the resistance of the metallic layered structures. The resistance of the structure determines the logical value to be stored therein.
Currently this technology is predominantly used in the disk drives. Disk drives use discs which are coated with a magnetizable medium for storage of digital information in a plurality of concentric data tracks. A track is a concentric set of magnetic bits on the disk. A sector is a part of each track that is defined with a magnetic marking and an ID number. A cylinder is group of tracks with the same radius. In a typical magnetic disk drive, a magnetic disk rotates at high speed and a read-write head uses air pressure to “fly” over the top surface of the disk. The head records information on the surface of the disk by impressing a magnetic field on the disk. Information is read back using the head by detecting magnetization of the disk surface. To access the disk requires a sequence of steps. The total time required to complete such sequence of steps is generally known as the access time. Access time has two major components: seek time and rotational latency. Seek time is the time needed for the read-write head to move radially to the cylinder containing the desired sector. Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. The access time is a sum of the seek time and the rotational latency time. The disk drives available today has an access time of 14 ms and this is too long for the future demand. The speed of the disk drive is negatively impacted by such long access time.
Spindle motors are generally used to rotate the disk at high speeds. A read-write head carried by a head slider is positioned over a track on the surface of the disk to write data to or read data from the track. The head slider is supported by a movable actuator which is controlled to position the read-write head carried by the head slider to a location with respect to the disk while the disk is rotating. However this arrangement using spindle motors are known to have problems. Even minor vibrations or bumps can cause the disk drive to crash. Mechanical constraints are limiting the function of the disk drive. Moreover, as the disk drives are being produced with increasing track densities and decreasing access time, the feedback control systems in modern disc drives must move the sliders to the correct position in a very short period of time. Seek errors may occur if the slider is not moved to the correct position.
As can be seen there is a clear need in the industry to have disk drives with shorter access time and without the mechanical constraints.
Meanwhile, a magnetic memory device known as Magnetoresistive Random Access Memory (MRAM) has been developed on an Integrated Circuit (IC) chip. This type of memory device generally includes conductive lines positioned perpendicular to one another. Each conductive lines act as either write or a bit line. A magnetic stack is placed where the two conductive lines cross. An electrical current flowing through one of the conductive line induces a magnetic field around that conductive line. A different current flowing through the other conductive line induces another magnetic field around the second conductive line. The induced magnetic fields align or realign the magnetic dipoles in the magnetic stack. The resistance of the magnetic stack determines the logical value to be stored therein.
For the MRAM the transistor logic circuits are embedded in the IC chip itself. As a a result of having transistor logic circuit in close proximity with the magnetic stack, the magnetic field interferes with the functions of the logic circuit. The magnetic field interference with the control circuits also makes it difficult to integrate MRAM various devices. Moreover, the amount of memory available through the use of an MRAM is only in the range of 1 Mbit. This amount of memory is not suitable for most applications. Also, since the MRAM is basically an IC chip it is not adaptable to other types of fabrications especially into memory devices like a disk drive.
While MRAMs provide a non-volatile memory it is not suitable for most of the present day applications due to its small amount of memory and inability to integrate, small amount of memory.
As can be seen there is clear need in the industry to have a memory device that is fast with a large memory and is durable.